One of the pathological hallmarks of Alzheimer's Disease (AD) is the senile plaque, principally composed of the A? peptide. A?1-42, the 42 amino-acid peptide fragment of the amyloid precursor protein (APP), has a striking propensity to aggregate into amyloid fibrils within plaques. A? aggregation appears to be concentration-dependent;thus, mechanisms that regulate extracellular (or interstitial fluid, ISF) A? levels may be important for amyloid plaque formation. Under normal physiological conditions, A? exists in brain extracellular fluids (ISF and CSF). This steady-state level of A? is maintained by a balance between production and clearance. While genetic mutations that lead to familial AD point to A? over-production as a cause for disease, it has been suggested that the much more prevalent sporadic form of AD may be caused by impaired A? clearance. A growing list of proteases are known to degrade soluble A? (sA?) in vitro, and inhibition or deletion of some of these proteases in vivo have resulted in increased steady-state levels of brain A? in mice, including neprilysin (NEP), insulin-degrading enzyme (IDE), endothelin-converting enzyme (ECE). We have recently published that the extracellular protease, matrix metalloproteinase-9 (MMP-9), is capable of degrading sA? in addition to fibrillar A? (fA?) in vitro and compact amyloid plaques in situ. The ability to degrade fA? is an important characteristic of MMP-9 which is lacking in most other A?-degrading proteases. MMP-9 immunoreactivity is seen in reactive astrocytes surrounding amyloid plaques of aged APP/PS1 mice. Furthermore, we have recently found that gene deletion of mmp9 in APP/PS1 mice resulted in an increase in amyloid plaque load. In this grant application, we will test the hypothesis that MMP-9 directly degrades extracellular A?, thus attenuating plaque pathogenesis in APP/PS1 mice. Furthermore, we will test the hypothesis that inhibiting TIMP-1, an endogenous inhibitor of MMP-9 (i.e. disinhibiting MMP-9), will further attenuate plaque pathogenesis. If true, TIMP-1 may be identified as a potential therapeutic target for the treatment of AD. In Aim 1, we will determine if MMP-9 activity attenuates amyloid plaque accumulation by directly degrading A? in APP/PS1 mice. In Aim 2, we will determine if MMP-9 directly catabolizes brain extracellular A?, using in vivo microdialysis in APP/PS1. In Aim 3, we will determine if MMP-9 regulates the growth/regression of pre- existing compact plaques in APP/PS1 mice imaged using intravital 2-photon microscopy. In Aim 4, we will determine if TIMP-1 inhibits MMP degradation of A2, accelerating amyloid plaque formation. If true, TIMP-1 may be identified as a potential therapeutic target for the treatment of AD. PUBLIC HEALTH RELEVANCE: This R01 grant application proposes to study the role of the protease MMP-9 and its endogenous inhibitor, TIMP-1 in regulating amyloid plaque growth in a mouse model of Alzheimer's disease. The central hypothesis of the grant is that MMP-9, secreted by astrocytes surrounding amyloid plaques, regulates amyloid plaque growth/regression by degrading soluble and fibrillar A? and amyloid fibrils of which plaques are comprised. If the hypothesis is true, potential targets for intervention to degrade plaques may be identified.